Democratic leaders took on the epic challenge of getting the US Senate to act on global warming today with the formal unveiling of a bill proposing an ambitious 20% cut in greenhouse gas emissions.

Senate Democrats turned out in strength for today's launch, which was seen at home and abroad as a crucial moment for advancing Barack Obama's agenda.

But the difficulties ahead were underscored by the senators' deep reluctance to use the words 'climate change' — even in the title of the bill, which is called the Clean Energy Jobs and American Power Act.

Instead, senators John Kerry and Barbara Boxer couched their bill as a chance for America to reclaim its energy independence by reducing oil imports.

"This is a security bill that puts Americans back in charge of our energy future," said Kerry. "It is our country's defence against the harms of pollution and the security risks of global climate change."

Today's bill sets out a more ambitious target for cutting greenhouse gas emissions than the 17% cut from 2005 levels by 2020 passed by the House of Representatives in June.

The draft would push for a 20% reduction from 2005 levels by 2020 and an 83% reduction by 2050.

The targets appear chosen for their resonance with European and Asian leaders.

International negotiators have been looking to America to demonstrate commitment to action on climate change ahead of a meeting at Copenhagen in December cast by the United Nations as a last chance to avoid catastrophic warming.

Otherwise, the bill appeared constructed according to advice of pollsters, which have regularly advised Democrats to cloak climate change action in the language of green job creation and national security.

In a summary released today, Kerry and Boxer said their bill was aimed at "putting America back in control of our energy future, reasserting American economic leadership and competitiveness, protecting our families from pollution, and ensuring our national security".

They began by laying out $10bn (£6.3bn) over 10 years for research into carbon capture technology and for pledging a greater role for nuclear power in America's energy mix.

The summary promised "robust border measures" against states which do not act to cut carbon emissions, presumably China and India.

It also sought to downplay the significance of its emissions cuts, arguing that fewer than 2% of American businesses would be affected.

"The system applies only to the largest polluters in the country - initially around 7,500 facilities that account for nearly three-quarters of US carbon pollution. Over 98% of American businesses, including farmers, are not covered by this programme," the summary said.

Despite such manoeuvring, it is far from clear that the Senate will be able to pivot from its battles over healthcare to climate change and make significant progress before the Copenhagen meeting.

The 800-page draft bill, which is still being worked on, is almost certain to undergo significant changes in the coming weeks with Democrats struggling to build support even from within their own ranks.

"Complex processes are part and parcel of passing major legislation," Tony Kreindler of the Environmental Defence Fund said. "The most important thing is that the draft be taken for what it is: a starting point that Senators can work with, tailor and pass."

Republicans, who are expected to largely oppose the bill, organised their own counter-press conference today.